SB 743921 is a second generation, highly potent and active KSP inhibitor with a Ki of 0.1 nM for KSP (Eg5). SB-743921 selectively inhibits kinesin spindle protein (KSP), an important protein involved in the early stages of mitosis that is expressed in proliferating cells. Inhibition of KSP results in inhibition of mitotic spindle assembly and interrupts cell division, thereby causing cell cycle arrest and induction of apoptosis. [1]